1. Field of the Invention
The present invention relates generally to an apparatus for sensing toner concentration in a container of developer material, and more particularly, to a technique employing a programmable active magnetic force to compress a preselected volume of developer material so that a representative permeability measurement of the developer material in the container can be obtained. The temperature of the developer material is also sensed using the device developing the magnetic force.
2. Description of the Prior Art
In an electrophotographic printing machine, the photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing marking or toner particles into contact therewith. This forms a powder image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the marking particles thereto in image configuration.
Various types of development systems have herein before been employed. These systems utilize two (2) component developer mixes or single component developer materials. Typical two component developer mixes employed are well known in the art, and generally comprise dyed or colored thermoplastic powders, known in the art as toner particles, which are mixed with coarser carrier granules, such as ferromagnetic granules. The toner particles and carrier granules are selected such that the toner particles acquire the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. When the developer mix is brought into contact with the charged photoconductive surface, the greater attractive force of the electrostatic latent image recorded thereon causes the toner particles to transfer from the carrier granules and adhere to the electrostatic latent image.
Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner particles of a color complimentary thereto. This process is repeated a plurality of cycles for differently colored images and their respective complimentary colored toner particles. For example, a red filtered light image is developed with cyan toner particles, while a green filtered light image is developed with magenta toner particles and a blue filtered light image with yellow toner particles. Each single color toner powder image is transferred to the copy sheet superimposed over the prior toner powder image. This creates a multi-layered toner powder image on the copy sheet. Thereafter, the multi-layered toner powder image is permanently affixed to the copy sheet creating a color copy. An illustrative electrophotographic printing machine for producing color copies is the Model No. 1005 made by the Xerox Corporation.
It is evident that in printing machines of this type, toner particles are depleted from the developer mixture. As the concentration of toner particles decreases, the density of the resultant copy degrades. In order to maintain the copies being reproduced at a specified minimum density, it is necessary to regulate the concentration of toner particles in the developer mixture. Moreover, sensing of toner concentration provides valuable input for process control at the development station as well as other stations of the electrophotographic printing machine. Toner concentration can be regulated by various known techniques, one of which includes monitoring an electro-magnetic property of the developer, such as permeability, permitivity or conductivity, to obtain information regarding the carrier-toner ratio.
A number of attempts have been made in the past to sense and control toner concentration in the developer mixture using "passive" magnetic sensors. This type of sensor is capable of determining toner concentration by measuring the magnetic permeability of developer flowing through a tube or the like. U.S. Pat. No. 3,572,551 discloses an apparatus for monitoring and controlling the concentration of toner in a developer mix. U.S. Pat. No. 3,698,926 discloses a method and apparatus for supplementing toner in electrophotographic machines. U.S. Pat. No. 3,802,381 discloses an apparatus for measuring the ratio of ferromagnetic carrier particles to toner particles in an electrostatic printing machine. U.S. Pat. No. 3,970,036 discloses a toner concentration detector in which developer removed from a photoreceptive member after developing is directed through a duct. Lastly, Xerox Disclosure Journal, Vol. 5, No. 3 at page 315 discloses a toner concentration meter system comprising a tube located in the air core of a transformer.
It has been found that in the above-cited references the passive sensors have a sensitivity to developer flow variations, and accordingly are subject to undesirable levels of "noise" or error. Other problems, such as developer aging, non-geometric packing fractions and changes in the environment also have an adverse effect on the performance of such passive sensors. U.S. Pat. No. 3,707,134 proposes an alternative to the passive sensors whereby an apparatus is provided for monitoring and controlling the ratio of toner-to-carrier particles of a developer mix. The apparatus includes an inductive sensing coil having an iron core. The coil is placed in the surroundings of the developer apparatus of an electrostatic copying machine so as to be in contact relation with the developer mix containing toner and magnetizable carrier particles. The inductive reactance of the coil is a function of the ratio of magnetizable particles per toner particles in the mix. Thus, as the toner is depleted, the inductance of the coil changes. The frequency of an oscillator circuit connected to the coil changes as the inductance of the coil is varied. The change in frequency produces a corresponding output to additional circuity which in turn operates a toner dispenser unit, causing toner to be added to the mix to restore the toner-to-carrier ratio to a predetermined level.
While the sensing arrangement of U.S. Pat. No. 3,707,134 identified above avoids some of the above-mentioned problems in connection with the passive sensors, it is relatively complex in design, and can yield inaccurate results. In particular, the sensor of this arrangement is positioned adjacent a magnetic brush and can thus become contaminated by stray developer material. Moreover, unless the layer of developer on the brush is closely metered, inaccurate toner concentration readings can be expected. Finally, the circuitry for the arrangement identified in that patent includes many components, and is thus relatively expensive to manufacture.
Another "active" magnetic sensor has been proposed in U.S. Pat. No. 5,166,729. This sensor is for use with a developer container adapted to retain a quantity of developer material therein, the developer material including varying concentrations of a magnetic carrier material and toner material. The toner concentration sensing apparatus comprises a device for generating a magnetic field within the developer container. The apparatus further comprises a device for controlling the generating device to selectively generate the magnetic field within the developer container, whereat a preselected portion of the developer material is compressed or otherwise compacted by the magnetic field. A signal is then generated across the generating device. The signal varies as a function of the concentration of the toner material.
While the above-identified prior sensing apparatus represents a significant improvement over the "passive" magnetic sensors, it would be useful to provide a flexible and programmable interface between the sensors and the main control system of the electrophotographic printing machine. For color machines, a single flexible interface would connect the plurality of individual sensors to the printing machine main control system.
It would further be desirable to provide a sensing apparatus that is capable of measuring magnetic permeability of developer material without being subjected to undesirable levels of noise arising in analog signal lines to the main control. Moreover, it would be desirable to provide a relatively inexpensive yet flexible sensing apparatus that is both easy to implement and programmable on-the-fly, using software parameters downloaded from a host in order to adapt to various operating conditions and environmental changes which may affect the operation of the electrophotographic printing machine. Such an apparatus including downloaded software parameters would simplify retrofitting procedures.
It would also be desirable to provide the above sensing apparatus with the further capability to sense the temperature of the toner or developer material with little or no additional hardware overhead.